Pain of any type is the most common reason for physician consultation in the U.S., prompting half of all Americans to seek medical care annually. It is a major symptom in many medical conditions, significantly interfering with a person's quality of life and general functioning. Diagnosis is based on characterizing pain in various ways, according to duration, intensity, type (dull, burning, throbbing or stabbing), source, or location in body. Usually if pain stops without treatment or responds to simple measures such as resting or taking an analgesic, it is then called ‘acute’ pain. But it may also become intractable and develop into a condition called chronic pain in which pain is no longer considered a symptom but an illness by itself.
The application of specific electrical energy to the spinal cord for the purpose of managing pain has been actively practiced since the 1960s. It is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nervous tissue. Such masking is known as paresthesia, a subjective sensation of numbness or tingling in the afflicted bodily regions. Such electrical stimulation of the spinal cord, once known as dorsal column stimulation, is now referred to as spinal cord stimulation or SCS.
FIGS. 1A-1B illustrate conventional placement of an SCS system 10. Conventional SCS systems include an implantable power source or implantable pulse generator (IPG) 12 and an implantable lead 14. Such PGs 12 are similar in size and weight to cardiac pacemakers and are typically implanted in the buttocks or abdomen of a patient P. Using fluoroscopy, the lead 14 is implanted into the epidural space E of the spinal column and positioned against the dura layer D of the spinal cord S, as illustrated in FIG. 1B. The lead 14 is implanted either through the skin via an epidural needle (for percutaneous leads) or directly and surgically through a mini laminotomy operation (for paddle leads or percutaneous leads). A laminotomy is a neurosurgical procedure that removes part of a lamina of the vertebral arch. The laminotomy creates an opening in the bone large enough to pass one or more leads through.
FIG. 2 illustrates example conventional paddle leads 16 and percutaneous leads 18. Paddle leads 16 typically have the form of a slab of silicon rubber having one or more electrodes 20 on its surface. Example dimensions of a paddle lead 16 are illustrated in FIG. 3. Percutaneous leads 18 typically have the form of a tube or rod having one or more electrodes 20 extending therearound. Example dimensions of a percutaneous lead 18 are illustrated in FIG. 4.
Implantation of a percutaneous lead 18 typically involves an incision over the low back area (for control of back and leg pain) or over the upper back and neck area (for pain in the arms). An epidural needle is placed through the incision into the epidural space and the lead is advanced and steered over the spinal cord until it reaches the area of the spinal cord that, when electrically stimulated, produces a tingling sensation (paresthesia) that covers the patient's painful area. To locate this area, the lead is moved and turned on and off while the patient provides feedback about stimulation coverage. Because the patient participates in this operation and directs the operator to the correct area of the spinal cord, the procedure is performed with conscious sedation.
Implantation of paddle leads 16 typically involves performing a mini laminotomy to implant the lead. An incision is made either slightly below or above the spinal cord segment to be stimulated. The epidural space is entered directly through the opening in the bone and a paddle lead 16 is placed over the region to stimulate the spinal cord. The target region for stimulation usually has been located before this procedure during a spinal cord stimulation trial with percutaneous leads 18.
Although such SCS systems have effectively relieved pain in some patients, these systems have a number of drawbacks. To begin, as illustrated in FIG. 5, the lead 14 is positioned upon the spinal cord dura layer D so that the electrodes 20 stimulate a wide portion of the spinal cord and associated spinal nervous tissue (as indicated by perimeter 21). The spinal cord is a continuous body and three spinal levels of the spinal cord are illustrated. For purposes of illustration, spinal levels are sub-sections of the spinal cord S depicting that portion where the dorsal root DR and ventral root VR join the spinal cord S. The spinal nerve N divides into the dorsal root DR and the dorsal root ganglion DRG and the ventral nerve root VR each of which feed into the spinal cord S. Generally, the dorsal roots DR feed into the posterior side of the spinal cord S and the ventral roots VR feed into the anterior side of the spinal cord S. For simplicity, each level shown illustrates the nerves of only one side and a normal anatomical configuration would have similar nerves on the opposite side of the spinal cord.
FIG. 6 illustrates a cross-sectional view of the lead 14 of FIG. 5 at a spinal level. Thus, as shown, the lead 14 is positioned against the dura layer D near the midline of the spinal cord S. The electrode 20 stimulates a wide portion of the spinal cord. In this example, the lead 14 is a unidirectional paddle lead so the stimulation energy 15 (indicated by perimeter 21) extends to one side of the lead 14. Significant energy 15 is utilized to penetrate the dura layer D and cerebral spinal fluid CSF to activate fibers in the spinal column extending within the posterior side of the spinal cord S, post-synaptically to the dorsal roots. And, in cases of omnidirectional leads, even more energy may be required due to loss of energy that is directed away from the target. Sensory spinal nervous tissue, or nervous tissue from the dorsal nerve roots, transmit pain signals. Therefore, such stimulation is intended to block the transmission of pain signals to the brain with the production of a tingling sensation (paresthesia) that masks the patient's sensation of pain. However, excessive tingling may be considered undesirable. Further, the energy 15 also typically penetrates the anterior side of the spinal cord S, stimulating the ventral horns, and consequently the ventral roots extending within the anterior side of the spinal cord S. Motor spinal nervous tissue, or nervous tissue from ventral nerve roots, transmits muscle/motor control signals. Therefore, electrical stimulation by the lead 14 often causes undesirable stimulation of the motor nerves in addition to the sensory spinal nervous tissue. The result is undesirable muscle contraction.
Because the electrodes span several levels and because they stimulate medial to spinal root entry points, the generated stimulation energy 15 stimulates or is applied to more than one type of nerve tissue on more than one level. Moreover, these and other conventional, non-specific stimulation systems also apply stimulation energy to the spinal cord and to other neural tissue beyond the intended stimulation targets. As used herein, non-specific stimulation refers to the fact that the stimulation energy is provided to multiple spinal levels including the nerves and the spinal cord generally and indiscriminately. This is the case even with the use of programmable electrode configurations wherein only a subset of the electrodes are used for stimulation. In fact, even if the epidural electrode is reduced in size to simply stimulate only one level, that electrode will apply stimulation energy non-specifically and indiscriminately (i.e. to many or all nerve fibers and other tissues) within the range of the applied energy.
Therefore, improved stimulation systems, devices and methods are desired that enable more precise and effective delivery of stimulation energy. At least some of these objectives will be met by the present invention.